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package java.lang.invoke;


import java.util.*;

import static java.lang.invoke.MethodHandleStatics.*;

/**
 * A method handle is a typed, directly executable reference to an underlying method, constructor,
 * field, or similar low-level operation, with optional transformations of arguments or return
 * values. These transformations are quite general, and include such patterns as {@linkplain #asType
 * conversion}, {@linkplain #bindTo insertion}, {@linkplain java.lang.invoke.MethodHandles#dropArguments
 * deletion}, and {@linkplain java.lang.invoke.MethodHandles#filterArguments substitution}.
 *
 * <h1>Method handle contents</h1> Method handles are dynamically and strongly typed according to
 * their parameter and return types. They are not distinguished by the name or the defining class of
 * their underlying methods. A method handle must be invoked using a symbolic type descriptor which
 * matches the method handle's own {@linkplain #type type descriptor}. <p> Every method handle
 * reports its type descriptor via the {@link #type type} accessor. This type descriptor is a {@link
 * java.lang.invoke.MethodType MethodType} object, whose structure is a series of classes, one of
 * which is the return type of the method (or {@code void.class} if none). <p> A method handle's
 * type controls the types of invocations it accepts, and the kinds of transformations that apply to
 * it. <p> A method handle contains a pair of special invoker methods called {@link #invokeExact
 * invokeExact} and {@link #invoke invoke}. Both invoker methods provide direct access to the method
 * handle's underlying method, constructor, field, or other operation, as modified by
 * transformations of arguments and return values. Both invokers accept calls which exactly match
 * the method handle's own type. The plain, inexact invoker also accepts a range of other call
 * types. <p> Method handles are immutable and have no visible state. Of course, they can be bound
 * to underlying methods or data which exhibit state. With respect to the Java Memory Model, any
 * method handle will behave as if all of its (internal) fields are final variables.  This means
 * that any method handle made visible to the application will always be fully formed. This is true
 * even if the method handle is published through a shared variable in a data race. <p> Method
 * handles cannot be subclassed by the user. Implementations may (or may not) create internal
 * subclasses of {@code MethodHandle} which may be visible via the {@link java.lang.Object#getClass
 * Object.getClass} operation.  The programmer should not draw conclusions about a method handle
 * from its specific class, as the method handle class hierarchy (if any) may change from time to
 * time or across implementations from different vendors.
 *
 * <h1>Method handle compilation</h1> A Java method call expression naming {@code invokeExact} or
 * {@code invoke} can invoke a method handle from Java source code. From the viewpoint of source
 * code, these methods can take any arguments and their result can be cast to any return type.
 * Formally this is accomplished by giving the invoker methods {@code Object} return types and
 * variable arity {@code Object} arguments, but they have an additional quality called <em>signature
 * polymorphism</em> which connects this freedom of invocation directly to the JVM execution stack.
 * <p> As is usual with virtual methods, source-level calls to {@code invokeExact} and {@code
 * invoke} compile to an {@code invokevirtual} instruction. More unusually, the compiler must record
 * the actual argument types, and may not perform method invocation conversions on the arguments.
 * Instead, it must push them on the stack according to their own unconverted types. The method
 * handle object itself is pushed on the stack before the arguments. The compiler then calls the
 * method handle with a symbolic type descriptor which describes the argument and return types. <p>
 * To issue a complete symbolic type descriptor, the compiler must also determine the return type.
 * This is based on a cast on the method invocation expression, if there is one, or else {@code
 * Object} if the invocation is an expression or else {@code void} if the invocation is a statement.
 * The cast may be to a primitive type (but not {@code void}). <p> As a corner case, an uncasted
 * {@code null} argument is given a symbolic type descriptor of {@code java.lang.Void}. The
 * ambiguity with the type {@code Void} is harmless, since there are no references of type {@code
 * Void} except the null reference.
 *
 * <h1>Method handle invocation</h1> The first time a {@code invokevirtual} instruction is executed
 * it is linked, by symbolically resolving the names in the instruction and verifying that the
 * method call is statically legal. This is true of calls to {@code invokeExact} and {@code invoke}.
 * In this case, the symbolic type descriptor emitted by the compiler is checked for correct syntax
 * and names it contains are resolved. Thus, an {@code invokevirtual} instruction which invokes a
 * method handle will always link, as long as the symbolic type descriptor is syntactically
 * well-formed and the types exist. <p> When the {@code invokevirtual} is executed after linking,
 * the receiving method handle's type is first checked by the JVM to ensure that it matches the
 * symbolic type descriptor. If the type match fails, it means that the method which the caller is
 * invoking is not present on the individual method handle being invoked. <p> In the case of {@code
 * invokeExact}, the type descriptor of the invocation (after resolving symbolic type names) must
 * exactly match the method type of the receiving method handle. In the case of plain, inexact
 * {@code invoke}, the resolved type descriptor must be a valid argument to the receiver's {@link
 * #asType asType} method. Thus, plain {@code invoke} is more permissive than {@code invokeExact}.
 * <p> After type matching, a call to {@code invokeExact} directly and immediately invoke the method
 * handle's underlying method (or other behavior, as the case may be). <p> A call to plain {@code
 * invoke} works the same as a call to {@code invokeExact}, if the symbolic type descriptor
 * specified by the caller exactly matches the method handle's own type. If there is a type
 * mismatch, {@code invoke} attempts to adjust the type of the receiving method handle, as if by a
 * call to {@link #asType asType}, to obtain an exactly invokable method handle {@code M2}. This
 * allows a more powerful negotiation of method type between caller and callee. <p> (<em>Note:</em>
 * The adjusted method handle {@code M2} is not directly observable, and implementations are
 * therefore not required to materialize it.)
 *
 * <h1>Invocation checking</h1> In typical programs, method handle type matching will usually
 * succeed. But if a match fails, the JVM will throw a {@link WrongMethodTypeException}, either
 * directly (in the case of {@code invokeExact}) or indirectly as if by a failed call to {@code
 * asType} (in the case of {@code invoke}). <p> Thus, a method type mismatch which might show up as
 * a linkage error in a statically typed program can show up as a dynamic {@code
 * WrongMethodTypeException} in a program which uses method handles. <p> Because method types
 * contain "live" {@code Class} objects, method type matching takes into account both types names
 * and class loaders. Thus, even if a method handle {@code M} is created in one class loader {@code
 * L1} and used in another {@code L2}, method handle calls are type-safe, because the caller's
 * symbolic type descriptor, as resolved in {@code L2}, is matched against the original callee
 * method's symbolic type descriptor, as resolved in {@code L1}. The resolution in {@code L1}
 * happens when {@code M} is created and its type is assigned, while the resolution in {@code L2}
 * happens when the {@code invokevirtual} instruction is linked. <p> Apart from the checking of type
 * descriptors, a method handle's capability to call its underlying method is unrestricted. If a
 * method handle is formed on a non-public method by a class that has access to that method, the
 * resulting handle can be used in any place by any caller who receives a reference to it. <p>
 * Unlike with the Core Reflection API, where access is checked every time a reflective method is
 * invoked, method handle access checking is performed <a href="MethodHandles.Lookup.html#access">when
 * the method handle is created</a>. In the case of {@code ldc} (see below), access checking is
 * performed as part of linking the constant pool entry underlying the constant method handle. <p>
 * Thus, handles to non-public methods, or to methods in non-public classes, should generally be
 * kept secret. They should not be passed to untrusted code unless their use from the untrusted code
 * would be harmless.
 *
 * <h1>Method handle creation</h1> Java code can create a method handle that directly accesses any
 * method, constructor, or field that is accessible to that code. This is done via a reflective,
 * capability-based API called {@link java.lang.invoke.MethodHandles.Lookup MethodHandles.Lookup}
 * For example, a static method handle can be obtained from {@link java.lang.invoke.MethodHandles.Lookup#findStatic
 * Lookup.findStatic}. There are also conversion methods from Core Reflection API objects, such as
 * {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}. <p> Like classes and
 * strings, method handles that correspond to accessible fields, methods, and constructors can also
 * be represented directly in a class file's constant pool as constants to be loaded by {@code ldc}
 * bytecodes. A new type of constant pool entry, {@code CONSTANT_MethodHandle}, refers directly to
 * an associated {@code CONSTANT_Methodref}, {@code CONSTANT_InterfaceMethodref}, or {@code
 * CONSTANT_Fieldref} constant pool entry. (For full details on method handle constants, see
 * sections 4.4.8 and 5.4.3.5 of the Java Virtual Machine Specification.) <p> Method handles
 * produced by lookups or constant loads from methods or constructors with the variable arity
 * modifier bit ({@code 0x0080}) have a corresponding variable arity, as if they were defined with
 * the help of {@link #asVarargsCollector asVarargsCollector}. <p> A method reference may refer
 * either to a static or non-static method. In the non-static case, the method handle type includes
 * an explicit receiver argument, prepended before any other arguments. In the method handle's type,
 * the initial receiver argument is typed according to the class under which the method was
 * initially requested. (E.g., if a non-static method handle is obtained via {@code ldc}, the type
 * of the receiver is the class named in the constant pool entry.) <p> Method handle constants are
 * subject to the same link-time access checks their corresponding bytecode instructions, and the
 * {@code ldc} instruction will throw corresponding linkage errors if the bytecode behaviors would
 * throw such errors. <p> As a corollary of this, access to protected members is restricted to
 * receivers only of the accessing class, or one of its subclasses, and the accessing class must in
 * turn be a subclass (or package sibling) of the protected member's defining class. If a method
 * reference refers to a protected non-static method or field of a class outside the current
 * package, the receiver argument will be narrowed to the type of the accessing class. <p> When a
 * method handle to a virtual method is invoked, the method is always looked up in the receiver
 * (that is, the first argument). <p> A non-virtual method handle to a specific virtual method
 * implementation can also be created.  These do not perform virtual lookup based on receiver type.
 * Such a method handle simulates the effect of an {@code invokespecial} instruction to the same
 * method.
 *
 * <h1>Usage examples</h1> Here are some examples of usage:
 * <blockquote><pre>{@code
 * Object x, y; String s; int i;
 * MethodType mt; MethodHandle mh;
 * MethodHandles.Lookup lookup = MethodHandles.lookup();
 * // mt is (char,char)String
 * mt = MethodType.methodType(String.class, char.class, char.class);
 * mh = lookup.findVirtual(String.class, "replace", mt);
 * s = (String) mh.invokeExact("daddy",'d','n');
 * // invokeExact(Ljava/lang/String;CC)Ljava/lang/String;
 * assertEquals(s, "nanny");
 * // weakly typed invocation (using MHs.invoke)
 * s = (String) mh.invokeWithArguments("sappy", 'p', 'v');
 * assertEquals(s, "savvy");
 * // mt is (Object[])List
 * mt = MethodType.methodType(java.util.List.class, Object[].class);
 * mh = lookup.findStatic(java.util.Arrays.class, "asList", mt);
 * assert(mh.isVarargsCollector());
 * x = mh.invoke("one", "two");
 * // invoke(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/Object;
 * assertEquals(x, java.util.Arrays.asList("one","two"));
 * // mt is (Object,Object,Object)Object
 * mt = MethodType.genericMethodType(3);
 * mh = mh.asType(mt);
 * x = mh.invokeExact((Object)1, (Object)2, (Object)3);
 * // invokeExact(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
 * assertEquals(x, java.util.Arrays.asList(1,2,3));
 * // mt is ()int
 * mt = MethodType.methodType(int.class);
 * mh = lookup.findVirtual(java.util.List.class, "size", mt);
 * i = (int) mh.invokeExact(java.util.Arrays.asList(1,2,3));
 * // invokeExact(Ljava/util/List;)I
 * assert(i == 3);
 * mt = MethodType.methodType(void.class, String.class);
 * mh = lookup.findVirtual(java.io.PrintStream.class, "println", mt);
 * mh.invokeExact(System.out, "Hello, world.");
 * // invokeExact(Ljava/io/PrintStream;Ljava/lang/String;)V
 * }</pre></blockquote>
 * Each of the above calls to {@code invokeExact} or plain {@code invoke} generates a single
 * invokevirtual instruction with the symbolic type descriptor indicated in the following comment.
 * In these examples, the helper method {@code assertEquals} is assumed to be a method which calls
 * {@link java.util.Objects#equals(Object, Object) Objects.equals} on its arguments, and asserts that
 * the result is true.
 *
 * <h1>Exceptions</h1> The methods {@code invokeExact} and {@code invoke} are declared to throw
 * {@link java.lang.Throwable Throwable}, which is to say that there is no static restriction on
 * what a method handle can throw.  Since the JVM does not distinguish between checked and unchecked
 * exceptions (other than by their class, of course), there is no particular effect on bytecode
 * shape from ascribing checked exceptions to method handle invocations.  But in Java source code,
 * methods which perform method handle calls must either explicitly throw {@code Throwable}, or else
 * must catch all throwables locally, rethrowing only those which are legal in the context, and
 * wrapping ones which are illegal.
 *
 * <h1><a name="sigpoly"></a>Signature polymorphism</h1> The unusual compilation and linkage
 * behavior of {@code invokeExact} and plain {@code invoke} is referenced by the term <em>signature
 * polymorphism</em>. As defined in the Java Language Specification, a signature polymorphic method
 * is one which can operate with any of a wide range of call signatures and return types. <p> In
 * source code, a call to a signature polymorphic method will compile, regardless of the requested
 * symbolic type descriptor. As usual, the Java compiler emits an {@code invokevirtual} instruction
 * with the given symbolic type descriptor against the named method. The unusual part is that the
 * symbolic type descriptor is derived from the actual argument and return types, not from the
 * method declaration. <p> When the JVM processes bytecode containing signature polymorphic calls,
 * it will successfully link any such call, regardless of its symbolic type descriptor. (In order to
 * retain type safety, the JVM will guard such calls with suitable dynamic type checks, as described
 * elsewhere.) <p> Bytecode generators, including the compiler back end, are required to emit
 * untransformed symbolic type descriptors for these methods. Tools which determine symbolic linkage
 * are required to accept such untransformed descriptors, without reporting linkage errors.
 *
 * <h1>Interoperation between method handles and the Core Reflection API</h1> Using factory methods
 * in the {@link java.lang.invoke.MethodHandles.Lookup Lookup} API, any class member represented by
 * a Core Reflection API object can be converted to a behaviorally equivalent method handle. For
 * example, a reflective {@link java.lang.reflect.Method Method} can be converted to a method handle
 * using {@link java.lang.invoke.MethodHandles.Lookup#unreflect Lookup.unreflect}. The resulting
 * method handles generally provide more direct and efficient access to the underlying class
 * members. <p> As a special case, when the Core Reflection API is used to view the signature
 * polymorphic methods {@code invokeExact} or plain {@code invoke} in this class, they appear as
 * ordinary non-polymorphic methods. Their reflective appearance, as viewed by {@link
 * java.lang.Class#getDeclaredMethod Class.getDeclaredMethod}, is unaffected by their special status
 * in this API. For example, {@link java.lang.reflect.Method#getModifiers Method.getModifiers} will
 * report exactly those modifier bits required for any similarly declared method, including in this
 * case {@code native} and {@code varargs} bits. <p> As with any reflected method, these methods
 * (when reflected) may be invoked via {@link java.lang.reflect.Method#invoke
 * java.lang.reflect.Method.invoke}. However, such reflective calls do not result in method handle
 * invocations. Such a call, if passed the required argument (a single one, of type {@code
 * Object[]}), will ignore the argument and will throw an {@code UnsupportedOperationException}. <p>
 * Since {@code invokevirtual} instructions can natively invoke method handles under any symbolic
 * type descriptor, this reflective view conflicts with the normal presentation of these methods via
 * bytecodes. Thus, these two native methods, when reflectively viewed by {@code
 * Class.getDeclaredMethod}, may be regarded as placeholders only. <p> In order to obtain an invoker
 * method for a particular type descriptor, use {@link java.lang.invoke.MethodHandles#exactInvoker
 * MethodHandles.exactInvoker}, or {@link java.lang.invoke.MethodHandles#invoker
 * MethodHandles.invoker}. The {@link java.lang.invoke.MethodHandles.Lookup#findVirtual
 * Lookup.findVirtual} API is also able to return a method handle to call {@code invokeExact} or
 * plain {@code invoke}, for any specified type descriptor .
 *
 * <h1>Interoperation between method handles and Java generics</h1> A method handle can be obtained
 * on a method, constructor, or field which is declared with Java generic types. As with the Core
 * Reflection API, the type of the method handle will constructed from the erasure of the
 * source-level type. When a method handle is invoked, the types of its arguments or the return
 * value cast type may be generic types or type instances. If this occurs, the compiler will replace
 * those types by their erasures when it constructs the symbolic type descriptor for the {@code
 * invokevirtual} instruction. <p> Method handles do not represent their function-like types in
 * terms of Java parameterized (generic) types, because there are three mismatches between
 * function-like types and parameterized Java types. <ul> <li>Method types range over all possible
 * arities, from no arguments to up to the  <a href="MethodHandle.html#maxarity">maximum number</a>
 * of allowed arguments. Generics are not variadic, and so cannot represent this.</li> <li>Method
 * types can specify arguments of primitive types, which Java generic types cannot range over.</li>
 * <li>Higher order functions over method handles (combinators) are often generic across a wide
 * range of function types, including those of multiple arities.  It is impossible to represent such
 * genericity with a Java type parameter.</li> </ul>
 *
 * <h1><a name="maxarity"></a>Arity limits</h1> The JVM imposes on all methods and constructors of
 * any kind an absolute limit of 255 stacked arguments.  This limit can appear more restrictive in
 * certain cases: <ul> <li>A {@code long} or {@code double} argument counts (for purposes of arity
 * limits) as two argument slots. <li>A non-static method consumes an extra argument for the object
 * on which the method is called. <li>A constructor consumes an extra argument for the object which
 * is being constructed. <li>Since a method handle&rsquo;s {@code invoke} method (or other
 * signature-polymorphic method) is non-virtual, it consumes an extra argument for the method handle
 * itself, in addition to any non-virtual receiver object. </ul> These limits imply that certain
 * method handles cannot be created, solely because of the JVM limit on stacked arguments. For
 * example, if a static JVM method accepts exactly 255 arguments, a method handle cannot be created
 * for it. Attempts to create method handles with impossible method types lead to an {@link
 * IllegalArgumentException}. In particular, a method handle&rsquo;s type must not have an arity of
 * the exact maximum 255.
 *
 * @author John Rose, JSR 292 EG
 * @see MethodType
 * @see MethodHandles
 */
public abstract class MethodHandle {

  static {
    MethodHandleImpl.initStatics();
  }

  /**
   * Internal marker interface which distinguishes (to the Java compiler)
   * those methods which are <a href="MethodHandle.html#sigpoly">signature polymorphic</a>.
   */
  @java.lang.annotation.Target({java.lang.annotation.ElementType.METHOD})
  @java.lang.annotation.Retention(java.lang.annotation.RetentionPolicy.RUNTIME)
  @interface PolymorphicSignature {

  }

  private final MethodType type;
  /*private*/ final LambdaForm form;
  // form is not private so that invokers can easily fetch it
    /*private*/ MethodHandle asTypeCache;
  // asTypeCache is not private so that invokers can easily fetch it
    /*non-public*/ byte customizationCount;
  // customizationCount should be accessible from invokers

  /**
   * Reports the type of this method handle.
   * Every invocation of this method handle via {@code invokeExact} must exactly match this type.
   *
   * @return the method handle type
   */
  public MethodType type() {
    return type;
  }

  /**
   * Package-private constructor for the method handle implementation hierarchy.
   * Method handle inheritance will be contained completely within
   * the {@code java.lang.invoke} package.
   */
  // @param type type (permanently assigned) of the new method handle
    /*non-public*/ MethodHandle(MethodType type, LambdaForm form) {
    type.getClass();  // explicit NPE
    form.getClass();  // explicit NPE
    this.type = type;
    this.form = form.uncustomize();

    this.form.prepare();  // TO DO:  Try to delay this step until just before invocation.
  }

  /**
   * Invokes the method handle, allowing any caller type descriptor, but requiring an exact type
   * match. The symbolic type descriptor at the call site of {@code invokeExact} must exactly match
   * this method handle's {@link #type type}. No conversions are allowed on arguments or return
   * values. <p> When this method is observed via the Core Reflection API, it will appear as a
   * single native method, taking an object array and returning an object. If this native method is
   * invoked directly via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke},
   * via JNI, or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect
   * Lookup.unreflect}, it will throw an {@code UnsupportedOperationException}.
   *
   * @param args the signature-polymorphic parameter list, statically represented using varargs
   * @return the signature-polymorphic result, statically represented using {@code Object}
   * @throws WrongMethodTypeException if the target's type is not identical with the caller's
   * symbolic type descriptor
   * @throws Throwable anything thrown by the underlying method propagates unchanged through the
   * method handle call
   */
  public final native @PolymorphicSignature
  Object invokeExact(Object... args) throws Throwable;

  /**
   * Invokes the method handle, allowing any caller type descriptor, and optionally performing
   * conversions on arguments and return values. <p> If the call site's symbolic type descriptor
   * exactly matches this method handle's {@link #type type}, the call proceeds as if by {@link
   * #invokeExact invokeExact}. <p> Otherwise, the call proceeds as if this method handle were first
   * adjusted by calling {@link #asType asType} to adjust this method handle to the required type,
   * and then the call proceeds as if by {@link #invokeExact invokeExact} on the adjusted method
   * handle. <p> There is no guarantee that the {@code asType} call is actually made. If the JVM can
   * predict the results of making the call, it may perform adaptations directly on the caller's
   * arguments, and call the target method handle according to its own exact type. <p> The resolved
   * type descriptor at the call site of {@code invoke} must be a valid argument to the receivers
   * {@code asType} method. In particular, the caller must specify the same argument arity as the
   * callee's type, if the callee is not a {@linkplain #asVarargsCollector variable arity
   * collector}. <p> When this method is observed via the Core Reflection API, it will appear as a
   * single native method, taking an object array and returning an object. If this native method is
   * invoked directly via {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke},
   * via JNI, or indirectly via {@link java.lang.invoke.MethodHandles.Lookup#unreflect
   * Lookup.unreflect}, it will throw an {@code UnsupportedOperationException}.
   *
   * @param args the signature-polymorphic parameter list, statically represented using varargs
   * @return the signature-polymorphic result, statically represented using {@code Object}
   * @throws WrongMethodTypeException if the target's type cannot be adjusted to the caller's
   * symbolic type descriptor
   * @throws ClassCastException if the target's type can be adjusted to the caller, but a reference
   * cast fails
   * @throws Throwable anything thrown by the underlying method propagates unchanged through the
   * method handle call
   */
  public final native @PolymorphicSignature
  Object invoke(Object... args) throws Throwable;

  /**
   * Private method for trusted invocation of a method handle respecting simplified signatures.
   * Type mismatches will not throw {@code WrongMethodTypeException}, but could crash the JVM.
   * <p>
   * The caller signature is restricted to the following basic types:
   * Object, int, long, float, double, and void return.
   * <p>
   * The caller is responsible for maintaining type correctness by ensuring
   * that the each outgoing argument value is a member of the range of the corresponding
   * callee argument type.
   * (The caller should therefore issue appropriate casts and integer narrowing
   * operations on outgoing argument values.)
   * The caller can assume that the incoming result value is part of the range
   * of the callee's return type.
   *
   * @param args the signature-polymorphic parameter list, statically represented using varargs
   * @return the signature-polymorphic result, statically represented using {@code Object}
   */
    /*non-public*/
  final native @PolymorphicSignature
  Object invokeBasic(Object... args) throws Throwable;

  /**
   * Private method for trusted invocation of a MemberName of kind {@code REF_invokeVirtual}.
   * The caller signature is restricted to basic types as with {@code invokeBasic}.
   * The trailing (not leading) argument must be a MemberName.
   *
   * @param args the signature-polymorphic parameter list, statically represented using varargs
   * @return the signature-polymorphic result, statically represented using {@code Object}
   */
    /*non-public*/
  static native @PolymorphicSignature
  Object linkToVirtual(Object... args) throws Throwable;

  /**
   * Private method for trusted invocation of a MemberName of kind {@code REF_invokeStatic}.
   * The caller signature is restricted to basic types as with {@code invokeBasic}.
   * The trailing (not leading) argument must be a MemberName.
   *
   * @param args the signature-polymorphic parameter list, statically represented using varargs
   * @return the signature-polymorphic result, statically represented using {@code Object}
   */
    /*non-public*/
  static native @PolymorphicSignature
  Object linkToStatic(Object... args) throws Throwable;

  /**
   * Private method for trusted invocation of a MemberName of kind {@code REF_invokeSpecial}.
   * The caller signature is restricted to basic types as with {@code invokeBasic}.
   * The trailing (not leading) argument must be a MemberName.
   *
   * @param args the signature-polymorphic parameter list, statically represented using varargs
   * @return the signature-polymorphic result, statically represented using {@code Object}
   */
    /*non-public*/
  static native @PolymorphicSignature
  Object linkToSpecial(Object... args) throws Throwable;

  /**
   * Private method for trusted invocation of a MemberName of kind {@code REF_invokeInterface}.
   * The caller signature is restricted to basic types as with {@code invokeBasic}.
   * The trailing (not leading) argument must be a MemberName.
   *
   * @param args the signature-polymorphic parameter list, statically represented using varargs
   * @return the signature-polymorphic result, statically represented using {@code Object}
   */
    /*non-public*/
  static native @PolymorphicSignature
  Object linkToInterface(Object... args) throws Throwable;

  /**
   * Performs a variable arity invocation, passing the arguments in the given list
   * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
   * which mentions only the type {@code Object}, and whose arity is the length
   * of the argument list.
   * <p>
   * Specifically, execution proceeds as if by the following steps,
   * although the methods are not guaranteed to be called if the JVM
   * can predict their effects.
   * <ul>
   * <li>Determine the length of the argument array as {@code N}.
   * For a null reference, {@code N=0}. </li>
   * <li>Determine the general type {@code TN} of {@code N} arguments as
   * as {@code TN=MethodType.genericMethodType(N)}.</li>
   * <li>Force the original target method handle {@code MH0} to the
   * required type, as {@code MH1 = MH0.asType(TN)}. </li>
   * <li>Spread the array into {@code N} separate arguments {@code A0, ...}. </li>
   * <li>Invoke the type-adjusted method handle on the unpacked arguments:
   * MH1.invokeExact(A0, ...). </li>
   * <li>Take the return value as an {@code Object} reference. </li>
   * </ul>
   * <p>
   * Because of the action of the {@code asType} step, the following argument
   * conversions are applied as necessary:
   * <ul>
   * <li>reference casting
   * <li>unboxing
   * <li>widening primitive conversions
   * </ul>
   * <p>
   * The result returned by the call is boxed if it is a primitive,
   * or forced to null if the return type is void.
   * <p>
   * This call is equivalent to the following code:
   * <blockquote><pre>{@code
   * MethodHandle invoker = MethodHandles.spreadInvoker(this.type(), 0);
   * Object result = invoker.invokeExact(this, arguments);
   * }</pre></blockquote>
   * <p>
   * Unlike the signature polymorphic methods {@code invokeExact} and {@code invoke},
   * {@code invokeWithArguments} can be accessed normally via the Core Reflection API and JNI.
   * It can therefore be used as a bridge between native or reflective code and method handles.
   *
   * @param arguments the arguments to pass to the target
   * @return the result returned by the target
   * @throws ClassCastException if an argument cannot be converted by reference casting
   * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given
   * number of {@code Object} arguments
   * @throws Throwable anything thrown by the target method invocation
   * @see MethodHandles#spreadInvoker
   */
  public Object invokeWithArguments(Object... arguments) throws Throwable {
    MethodType invocationType = MethodType
        .genericMethodType(arguments == null ? 0 : arguments.length);
    return invocationType.invokers().spreadInvoker(0)
        .invokeExact(asType(invocationType), arguments);
  }

  /**
   * Performs a variable arity invocation, passing the arguments in the given array
   * to the method handle, as if via an inexact {@link #invoke invoke} from a call site
   * which mentions only the type {@code Object}, and whose arity is the length
   * of the argument array.
   * <p>
   * This method is also equivalent to the following code:
   * <blockquote><pre>{@code
   *   invokeWithArguments(arguments.toArray()
   * }</pre></blockquote>
   *
   * @param arguments the arguments to pass to the target
   * @return the result returned by the target
   * @throws NullPointerException if {@code arguments} is a null reference
   * @throws ClassCastException if an argument cannot be converted by reference casting
   * @throws WrongMethodTypeException if the target's type cannot be adjusted to take the given
   * number of {@code Object} arguments
   * @throws Throwable anything thrown by the target method invocation
   */
  public Object invokeWithArguments(java.util.List<?> arguments) throws Throwable {
    return invokeWithArguments(arguments.toArray());
  }

  /**
   * Produces an adapter method handle which adapts the type of the current method handle to a new
   * type. The resulting method handle is guaranteed to report a type which is equal to the desired
   * new type. <p> If the original type and new type are equal, returns {@code this}. <p> The new
   * method handle, when invoked, will perform the following steps: <ul> <li>Convert the incoming
   * argument list to match the original method handle's argument list. <li>Invoke the original
   * method handle on the converted argument list. <li>Convert any result returned by the original
   * method handle to the return type of new method handle. </ul> <p> This method provides the
   * crucial behavioral difference between {@link #invokeExact invokeExact} and plain, inexact
   * {@link #invoke invoke}. The two methods perform the same steps when the caller's type
   * descriptor exactly m atches the callee's, but when the types differ, plain {@link #invoke
   * invoke} also calls {@code asType} (or some internal equivalent) in order to match up the
   * caller's and callee's types. <p> If the current method is a variable arity method handle
   * argument list conversion may involve the conversion and collection of several arguments into an
   * array, as {@linkplain #asVarargsCollector described elsewhere}. In every other case, all
   * conversions are applied <em>pairwise</em>, which means that each argument or return value is
   * converted to exactly one argument or return value (or no return value). The applied conversions
   * are defined by consulting the the corresponding component types of the old and new method
   * handle types. <p> Let <em>T0</em> and <em>T1</em> be corresponding new and old parameter types,
   * or old and new return types.  Specifically, for some valid index {@code i}, let
   * <em>T0</em>{@code =newType.parameterType(i)} and <em>T1</em>{@code
   * =this.type().parameterType(i)}. Or else, going the other way for return values, let
   * <em>T0</em>{@code =this.type().returnType()} and <em>T1</em>{@code =newType.returnType()}. If
   * the types are the same, the new method handle makes no change to the corresponding argument or
   * return value (if any). Otherwise, one of the following conversions is applied if possible: <ul>
   * <li>If <em>T0</em> and <em>T1</em> are references, then a cast to <em>T1</em> is applied. (The
   * types do not need to be related in any particular way. This is because a dynamic value of null
   * can convert to any reference type.) <li>If <em>T0</em> and <em>T1</em> are primitives, then a
   * Java method invocation conversion (JLS 5.3) is applied, if one exists. (Specifically,
   * <em>T0</em> must convert to <em>T1</em> by a widening primitive conversion.) <li>If <em>T0</em>
   * is a primitive and <em>T1</em> a reference, a Java casting conversion (JLS 5.5) is applied if
   * one exists. (Specifically, the value is boxed from <em>T0</em> to its wrapper class, which is
   * then widened as needed to <em>T1</em>.) <li>If <em>T0</em> is a reference and <em>T1</em> a
   * primitive, an unboxing conversion will be applied at runtime, possibly followed by a Java
   * method invocation conversion (JLS 5.3) on the primitive value.  (These are the primitive
   * widening conversions.) <em>T0</em> must be a wrapper class or a supertype of one. (In the case
   * where <em>T0</em> is Object, these are the conversions allowed by {@link
   * java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}.) The unboxing conversion must
   * have a possibility of success, which means that if <em>T0</em> is not itself a wrapper class,
   * there must exist at least one wrapper class <em>TW</em> which is a subtype of <em>T0</em> and
   * whose unboxed primitive value can be widened to <em>T1</em>. <li>If the return type <em>T1</em>
   * is marked as void, any returned value is discarded <li>If the return type <em>T0</em> is void
   * and <em>T1</em> a reference, a null value is introduced. <li>If the return type <em>T0</em> is
   * void and <em>T1</em> a primitive, a zero value is introduced. </ul> (<em>Note:</em> Both
   * <em>T0</em> and <em>T1</em> may be regarded as static types, because neither corresponds
   * specifically to the <em>dynamic type</em> of any actual argument or return value.) <p> The
   * method handle conversion cannot be made if any one of the required pairwise conversions cannot
   * be made. <p> At runtime, the conversions applied to reference arguments or return values may
   * require additional runtime checks which can fail. An unboxing operation may fail because the
   * original reference is null, causing a {@link java.lang.NullPointerException
   * NullPointerException}. An unboxing operation or a reference cast may also fail on a reference
   * to an object of the wrong type, causing a {@link java.lang.ClassCastException
   * ClassCastException}. Although an unboxing operation may accept several kinds of wrappers, if
   * none are available, a {@code ClassCastException} will be thrown.
   *
   * @param newType the expected type of the new method handle
   * @return a method handle which delegates to {@code this} after performing any necessary argument
   * conversions, and arranges for any necessary return value conversions
   * @throws NullPointerException if {@code newType} is a null reference
   * @throws WrongMethodTypeException if the conversion cannot be made
   * @see MethodHandles#explicitCastArguments
   */
  public MethodHandle asType(MethodType newType) {
    // Fast path alternative to a heavyweight {@code asType} call.
    // Return 'this' if the conversion will be a no-op.
    if (newType == type) {
      return this;
    }
    // Return 'this.asTypeCache' if the conversion is already memoized.
    MethodHandle atc = asTypeCached(newType);
    if (atc != null) {
      return atc;
    }
    return asTypeUncached(newType);
  }

  private MethodHandle asTypeCached(MethodType newType) {
    MethodHandle atc = asTypeCache;
    if (atc != null && newType == atc.type) {
      return atc;
    }
    return null;
  }

  /**
   * Override this to change asType behavior.
   */
    /*non-public*/ MethodHandle asTypeUncached(MethodType newType) {
    if (!type.isConvertibleTo(newType)) {
      throw new WrongMethodTypeException("cannot convert " + this + " to " + newType);
    }
    return asTypeCache = MethodHandleImpl.makePairwiseConvert(this, newType, true);
  }

  /**
   * Makes an <em>array-spreading</em> method handle, which accepts a trailing array argument
   * and spreads its elements as positional arguments.
   * The new method handle adapts, as its <i>target</i>,
   * the current method handle.  The type of the adapter will be
   * the same as the type of the target, except that the final
   * {@code arrayLength} parameters of the target's type are replaced
   * by a single array parameter of type {@code arrayType}.
   * <p>
   * If the array element type differs from any of the corresponding
   * argument types on the original target,
   * the original target is adapted to take the array elements directly,
   * as if by a call to {@link #asType asType}.
   * <p>
   * When called, the adapter replaces a trailing array argument
   * by the array's elements, each as its own argument to the target.
   * (The order of the arguments is preserved.)
   * They are converted pairwise by casting and/or unboxing
   * to the types of the trailing parameters of the target.
   * Finally the target is called.
   * What the target eventually returns is returned unchanged by the adapter.
   * <p>
   * Before calling the target, the adapter verifies that the array
   * contains exactly enough elements to provide a correct argument count
   * to the target method handle.
   * (The array may also be null when zero elements are required.)
   * <p>
   * If, when the adapter is called, the supplied array argument does
   * not have the correct number of elements, the adapter will throw
   * an {@link IllegalArgumentException} instead of invoking the target.
   * <p>
   * Here are some simple examples of array-spreading method handles:
   * <blockquote><pre>{@code
   * MethodHandle equals = publicLookup()
   * .findVirtual(String.class, "equals", methodType(boolean.class, Object.class));
   * assert( (boolean) equals.invokeExact("me", (Object)"me"));
   * assert(!(boolean) equals.invokeExact("me", (Object)"thee"));
   * // spread both arguments from a 2-array:
   * MethodHandle eq2 = equals.asSpreader(Object[].class, 2);
   * assert( (boolean) eq2.invokeExact(new Object[]{ "me", "me" }));
   * assert(!(boolean) eq2.invokeExact(new Object[]{ "me", "thee" }));
   * // try to spread from anything but a 2-array:
   * for (int n = 0; n <= 10; n++) {
   * Object[] badArityArgs = (n == 2 ? null : new Object[n]);
   * try { assert((boolean) eq2.invokeExact(badArityArgs) && false); }
   * catch (IllegalArgumentException ex) { } // OK
   * }
   * // spread both arguments from a String array:
   * MethodHandle eq2s = equals.asSpreader(String[].class, 2);
   * assert( (boolean) eq2s.invokeExact(new String[]{ "me", "me" }));
   * assert(!(boolean) eq2s.invokeExact(new String[]{ "me", "thee" }));
   * // spread second arguments from a 1-array:
   * MethodHandle eq1 = equals.asSpreader(Object[].class, 1);
   * assert( (boolean) eq1.invokeExact("me", new Object[]{ "me" }));
   * assert(!(boolean) eq1.invokeExact("me", new Object[]{ "thee" }));
   * // spread no arguments from a 0-array or null:
   * MethodHandle eq0 = equals.asSpreader(Object[].class, 0);
   * assert( (boolean) eq0.invokeExact("me", (Object)"me", new Object[0]));
   * assert(!(boolean) eq0.invokeExact("me", (Object)"thee", (Object[])null));
   * // asSpreader and asCollector are approximate inverses:
   * for (int n = 0; n <= 2; n++) {
   * for (Class<?> a : new Class<?>[]{Object[].class, String[].class, CharSequence[].class}) {
   * MethodHandle equals2 = equals.asSpreader(a, n).asCollector(a, n);
   * assert( (boolean) equals2.invokeWithArguments("me", "me"));
   * assert(!(boolean) equals2.invokeWithArguments("me", "thee"));
   * }
   * }
   * MethodHandle caToString = publicLookup()
   * .findStatic(Arrays.class, "toString", methodType(String.class, char[].class));
   * assertEquals("[A, B, C]", (String) caToString.invokeExact("ABC".toCharArray()));
   * MethodHandle caString3 = caToString.asCollector(char[].class, 3);
   * assertEquals("[A, B, C]", (String) caString3.invokeExact('A', 'B', 'C'));
   * MethodHandle caToString2 = caString3.asSpreader(char[].class, 2);
   * assertEquals("[A, B, C]", (String) caToString2.invokeExact('A', "BC".toCharArray()));
   * }</pre></blockquote>
   *
   * @param arrayType usually {@code Object[]}, the type of the array argument from which to extract
   * the spread arguments
   * @param arrayLength the number of arguments to spread from an incoming array argument
   * @return a new method handle which spreads its final array argument, before calling the original
   * method handle
   * @throws NullPointerException if {@code arrayType} is a null reference
   * @throws IllegalArgumentException if {@code arrayType} is not an array type, or if target does
   * not have at least {@code arrayLength} parameter types, or if {@code arrayLength} is negative,
   * or if the resulting method handle's type would have <a href="MethodHandle.html#maxarity">too
   * many parameters</a>
   * @throws WrongMethodTypeException if the implied {@code asType} call fails
   * @see #asCollector
   */
  public MethodHandle asSpreader(Class<?> arrayType, int arrayLength) {
    MethodType postSpreadType = asSpreaderChecks(arrayType, arrayLength);
    int arity = type().parameterCount();
    int spreadArgPos = arity - arrayLength;
    MethodHandle afterSpread = this.asType(postSpreadType);
    BoundMethodHandle mh = afterSpread.rebind();
    LambdaForm lform = mh.editor().spreadArgumentsForm(1 + spreadArgPos, arrayType, arrayLength);
    MethodType preSpreadType = postSpreadType.replaceParameterTypes(spreadArgPos, arity, arrayType);
    return mh.copyWith(preSpreadType, lform);
  }

  /**
   * See if {@code asSpreader} can be validly called with the given arguments.
   * Return the type of the method handle call after spreading but before conversions.
   */
  private MethodType asSpreaderChecks(Class<?> arrayType, int arrayLength) {
    spreadArrayChecks(arrayType, arrayLength);
    int nargs = type().parameterCount();
    if (nargs < arrayLength || arrayLength < 0) {
      throw newIllegalArgumentException("bad spread array length");
    }
    Class<?> arrayElement = arrayType.getComponentType();
    MethodType mtype = type();
    boolean match = true, fail = false;
    for (int i = nargs - arrayLength; i < nargs; i++) {
      Class<?> ptype = mtype.parameterType(i);
      if (ptype != arrayElement) {
        match = false;
        if (!MethodType.canConvert(arrayElement, ptype)) {
          fail = true;
          break;
        }
      }
    }
    if (match) {
      return mtype;
    }
    MethodType needType = mtype.asSpreaderType(arrayType, arrayLength);
    if (!fail) {
      return needType;
    }
    // elicit an error:
    this.asType(needType);
    throw newInternalError("should not return", null);
  }

  private void spreadArrayChecks(Class<?> arrayType, int arrayLength) {
    Class<?> arrayElement = arrayType.getComponentType();
    if (arrayElement == null) {
      throw newIllegalArgumentException("not an array type", arrayType);
    }
    if ((arrayLength & 0x7F) != arrayLength) {
      if ((arrayLength & 0xFF) != arrayLength) {
        throw newIllegalArgumentException("array length is not legal", arrayLength);
      }
      assert (arrayLength >= 128);
      if (arrayElement == long.class ||
          arrayElement == double.class) {
        throw newIllegalArgumentException("array length is not legal for long[] or double[]",
            arrayLength);
      }
    }
  }

  /**
   * Makes an <em>array-collecting</em> method handle, which accepts a given number of trailing
   * positional arguments and collects them into an array argument. The new method handle adapts, as
   * its <i>target</i>, the current method handle.  The type of the adapter will be the same as the
   * type of the target, except that a single trailing parameter (usually of type {@code arrayType})
   * is replaced by {@code arrayLength} parameters whose type is element type of {@code arrayType}.
   * <p> If the array type differs from the final argument type on the original target, the original
   * target is adapted to take the array type directly, as if by a call to {@link #asType asType}.
   * <p> When called, the adapter replaces its trailing {@code arrayLength} arguments by a single
   * new array of type {@code arrayType}, whose elements comprise (in order) the replaced arguments.
   * Finally the target is called. What the target eventually returns is returned unchanged by the
   * adapter. <p> (The array may also be a shared constant when {@code arrayLength} is zero.) <p>
   * (<em>Note:</em> The {@code arrayType} is often identical to the last parameter type of the
   * original target. It is an explicit argument for symmetry with {@code asSpreader}, and also to
   * allow the target to use a simple {@code Object} as its last parameter type.) <p> In order to
   * create a collecting adapter which is not restricted to a particular number of collected
   * arguments, use {@link #asVarargsCollector asVarargsCollector} instead. <p> Here are some
   * examples of array-collecting method handles:
   * <blockquote><pre>{@code
   * MethodHandle deepToString = publicLookup()
   * .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
   * assertEquals("[won]",   (String) deepToString.invokeExact(new Object[]{"won"}));
   * MethodHandle ts1 = deepToString.asCollector(Object[].class, 1);
   * assertEquals(methodType(String.class, Object.class), ts1.type());
   * //assertEquals("[won]", (String) ts1.invokeExact(         new Object[]{"won"})); //FAIL
   * assertEquals("[[won]]", (String) ts1.invokeExact((Object) new Object[]{"won"}));
   * // arrayType can be a subtype of Object[]
   * MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
   * assertEquals(methodType(String.class, String.class, String.class), ts2.type());
   * assertEquals("[two, too]", (String) ts2.invokeExact("two", "too"));
   * MethodHandle ts0 = deepToString.asCollector(Object[].class, 0);
   * assertEquals("[]", (String) ts0.invokeExact());
   * // collectors can be nested, Lisp-style
   * MethodHandle ts22 = deepToString.asCollector(Object[].class, 3).asCollector(String[].class,
   * 2);
   * assertEquals("[A, B, [C, D]]", ((String) ts22.invokeExact((Object)'A', (Object)"B", "C",
   * "D")));
   * // arrayType can be any primitive array type
   * MethodHandle bytesToString = publicLookup()
   * .findStatic(Arrays.class, "toString", methodType(String.class, byte[].class))
   * .asCollector(byte[].class, 3);
   * assertEquals("[1, 2, 3]", (String) bytesToString.invokeExact((byte)1, (byte)2, (byte)3));
   * MethodHandle longsToString = publicLookup()
   * .findStatic(Arrays.class, "toString", methodType(String.class, long[].class))
   * .asCollector(long[].class, 1);
   * assertEquals("[123]", (String) longsToString.invokeExact((long)123));
   * }</pre></blockquote>
   *
   * @param arrayType often {@code Object[]}, the type of the array argument which will collect the
   * arguments
   * @param arrayLength the number of arguments to collect into a new array argument
   * @return a new method handle which collects some trailing argument into an array, before calling
   * the original method handle
   * @throws NullPointerException if {@code arrayType} is a null reference
   * @throws IllegalArgumentException if {@code arrayType} is not an array type or {@code arrayType}
   * is not assignable to this method handle's trailing parameter type, or {@code arrayLength} is
   * not a legal array size, or the resulting method handle's type would have <a
   * href="MethodHandle.html#maxarity">too many parameters</a>
   * @throws WrongMethodTypeException if the implied {@code asType} call fails
   * @see #asSpreader
   * @see #asVarargsCollector
   */
  public MethodHandle asCollector(Class<?> arrayType, int arrayLength) {
    asCollectorChecks(arrayType, arrayLength);
    int collectArgPos = type().parameterCount() - 1;
    BoundMethodHandle mh = rebind();
    MethodType resultType = type().asCollectorType(arrayType, arrayLength);
    MethodHandle newArray = MethodHandleImpl.varargsArray(arrayType, arrayLength);
    LambdaForm lform = mh.editor().collectArgumentArrayForm(1 + collectArgPos, newArray);
    if (lform != null) {
      return mh.copyWith(resultType, lform);
    }
    lform = mh.editor().collectArgumentsForm(1 + collectArgPos, newArray.type().basicType());
    return mh.copyWithExtendL(resultType, lform, newArray);
  }

  /**
   * See if {@code asCollector} can be validly called with the given arguments.
   * Return false if the last parameter is not an exact match to arrayType.
   */
    /*non-public*/ boolean asCollectorChecks(Class<?> arrayType, int arrayLength) {
    spreadArrayChecks(arrayType, arrayLength);
    int nargs = type().parameterCount();
    if (nargs != 0) {
      Class<?> lastParam = type().parameterType(nargs - 1);
      if (lastParam == arrayType) {
        return true;
      }
      if (lastParam.isAssignableFrom(arrayType)) {
        return false;
      }
    }
    throw newIllegalArgumentException("array type not assignable to trailing argument", this,
        arrayType);
  }

  /**
   * Makes a <em>variable arity</em> adapter which is able to accept
   * any number of trailing positional arguments and collect them
   * into an array argument.
   * <p>
   * The type and behavior of the adapter will be the same as
   * the type and behavior of the target, except that certain
   * {@code invoke} and {@code asType} requests can lead to
   * trailing positional arguments being collected into target's
   * trailing parameter.
   * Also, the last parameter type of the adapter will be
   * {@code arrayType}, even if the target has a different
   * last parameter type.
   * <p>
   * This transformation may return {@code this} if the method handle is
   * already of variable arity and its trailing parameter type
   * is identical to {@code arrayType}.
   * <p>
   * When called with {@link #invokeExact invokeExact}, the adapter invokes
   * the target with no argument changes.
   * (<em>Note:</em> This behavior is different from a
   * {@linkplain #asCollector fixed arity collector},
   * since it accepts a whole array of indeterminate length,
   * rather than a fixed number of arguments.)
   * <p>
   * When called with plain, inexact {@link #invoke invoke}, if the caller
   * type is the same as the adapter, the adapter invokes the target as with
   * {@code invokeExact}.
   * (This is the normal behavior for {@code invoke} when types match.)
   * <p>
   * Otherwise, if the caller and adapter arity are the same, and the
   * trailing parameter type of the caller is a reference type identical to
   * or assignable to the trailing parameter type of the adapter,
   * the arguments and return values are converted pairwise,
   * as if by {@link #asType asType} on a fixed arity
   * method handle.
   * <p>
   * Otherwise, the arities differ, or the adapter's trailing parameter
   * type is not assignable from the corresponding caller type.
   * In this case, the adapter replaces all trailing arguments from
   * the original trailing argument position onward, by
   * a new array of type {@code arrayType}, whose elements
   * comprise (in order) the replaced arguments.
   * <p>
   * The caller type must provides as least enough arguments,
   * and of the correct type, to satisfy the target's requirement for
   * positional arguments before the trailing array argument.
   * Thus, the caller must supply, at a minimum, {@code N-1} arguments,
   * where {@code N} is the arity of the target.
   * Also, there must exist conversions from the incoming arguments
   * to the target's arguments.
   * As with other uses of plain {@code invoke}, if these basic
   * requirements are not fulfilled, a {@code WrongMethodTypeException}
   * may be thrown.
   * <p>
   * In all cases, what the target eventually returns is returned unchanged by the adapter.
   * <p>
   * In the final case, it is exactly as if the target method handle were
   * temporarily adapted with a {@linkplain #asCollector fixed arity collector}
   * to the arity required by the caller type.
   * (As with {@code asCollector}, if the array length is zero,
   * a shared constant may be used instead of a new array.
   * If the implied call to {@code asCollector} would throw
   * an {@code IllegalArgumentException} or {@code WrongMethodTypeException},
   * the call to the variable arity adapter must throw
   * {@code WrongMethodTypeException}.)
   * <p>
   * The behavior of {@link #asType asType} is also specialized for
   * variable arity adapters, to maintain the invariant that
   * plain, inexact {@code invoke} is always equivalent to an {@code asType}
   * call to adjust the target type, followed by {@code invokeExact}.
   * Therefore, a variable arity adapter responds
   * to an {@code asType} request by building a fixed arity collector,
   * if and only if the adapter and requested type differ either
   * in arity or trailing argument type.
   * The resulting fixed arity collector has its type further adjusted
   * (if necessary) to the requested type by pairwise conversion,
   * as if by another application of {@code asType}.
   * <p>
   * When a method handle is obtained by executing an {@code ldc} instruction
   * of a {@code CONSTANT_MethodHandle} constant, and the target method is marked
   * as a variable arity method (with the modifier bit {@code 0x0080}),
   * the method handle will accept multiple arities, as if the method handle
   * constant were created by means of a call to {@code asVarargsCollector}.
   * <p>
   * In order to create a collecting adapter which collects a predetermined
   * number of arguments, and whose type reflects this predetermined number,
   * use {@link #asCollector asCollector} instead.
   * <p>
   * No method handle transformations produce new method handles with
   * variable arity, unless they are documented as doing so.
   * Therefore, besides {@code asVarargsCollector},
   * all methods in {@code MethodHandle} and {@code MethodHandles}
   * will return a method handle with fixed arity,
   * except in the cases where they are specified to return their original
   * operand (e.g., {@code asType} of the method handle's own type).
   * <p>
   * Calling {@code asVarargsCollector} on a method handle which is already
   * of variable arity will produce a method handle with the same type and behavior.
   * It may (or may not) return the original variable arity method handle.
   * <p>
   * Here is an example, of a list-making variable arity method handle:
   * <blockquote><pre>{@code
   * MethodHandle deepToString = publicLookup()
   * .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
   * MethodHandle ts1 = deepToString.asVarargsCollector(Object[].class);
   * assertEquals("[won]",   (String) ts1.invokeExact(    new Object[]{"won"}));
   * assertEquals("[won]",   (String) ts1.invoke(         new Object[]{"won"}));
   * assertEquals("[won]",   (String) ts1.invoke(                      "won" ));
   * assertEquals("[[won]]", (String) ts1.invoke((Object) new Object[]{"won"}));
   * // findStatic of Arrays.asList(...) produces a variable arity method handle:
   * MethodHandle asList = publicLookup()
   * .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class));
   * assertEquals(methodType(List.class, Object[].class), asList.type());
   * assert(asList.isVarargsCollector());
   * assertEquals("[]", asList.invoke().toString());
   * assertEquals("[1]", asList.invoke(1).toString());
   * assertEquals("[two, too]", asList.invoke("two", "too").toString());
   * String[] argv = { "three", "thee", "tee" };
   * assertEquals("[three, thee, tee]", asList.invoke(argv).toString());
   * assertEquals("[three, thee, tee]", asList.invoke((Object[])argv).toString());
   * List ls = (List) asList.invoke((Object)argv);
   * assertEquals(1, ls.size());
   * assertEquals("[three, thee, tee]", Arrays.toString((Object[])ls.get(0)));
   * }</pre></blockquote>
   * <p style="font-size:smaller;">
   * <em>Discussion:</em>
   * These rules are designed as a dynamically-typed variation
   * of the Java rules for variable arity methods.
   * In both cases, callers to a variable arity method or method handle
   * can either pass zero or more positional arguments, or else pass
   * pre-collected arrays of any length.  Users should be aware of the
   * special role of the final argument, and of the effect of a
   * type match on that final argument, which determines whether
   * or not a single trailing argument is interpreted as a whole
   * array or a single element of an array to be collected.
   * Note that the dynamic type of the trailing argument has no
   * effect on this decision, only a comparison between the symbolic
   * type descriptor of the call site and the type descriptor of the method handle.)
   *
   * @param arrayType often {@code Object[]}, the type of the array argument which will collect the
   * arguments
   * @return a new method handle which can collect any number of trailing arguments into an array,
   * before calling the original method handle
   * @throws NullPointerException if {@code arrayType} is a null reference
   * @throws IllegalArgumentException if {@code arrayType} is not an array type or {@code arrayType}
   * is not assignable to this method handle's trailing parameter type
   * @see #asCollector
   * @see #isVarargsCollector
   * @see #asFixedArity
   */
  public MethodHandle asVarargsCollector(Class<?> arrayType) {
    arrayType.getClass(); // explicit NPE
    boolean lastMatch = asCollectorChecks(arrayType, 0);
    if (isVarargsCollector() && lastMatch) {
      return this;
    }
    return MethodHandleImpl.makeVarargsCollector(this, arrayType);
  }

  /**
   * Determines if this method handle
   * supports {@linkplain #asVarargsCollector variable arity} calls.
   * Such method handles arise from the following sources:
   * <ul>
   * <li>a call to {@linkplain #asVarargsCollector asVarargsCollector}
   * <li>a call to a {@linkplain java.lang.invoke.MethodHandles.Lookup lookup method}
   * which resolves to a variable arity Java method or constructor
   * <li>an {@code ldc} instruction of a {@code CONSTANT_MethodHandle}
   * which resolves to a variable arity Java method or constructor
   * </ul>
   *
   * @return true if this method handle accepts more than one arity of plain, inexact {@code invoke}
   * calls
   * @see #asVarargsCollector
   * @see #asFixedArity
   */
  public boolean isVarargsCollector() {
    return false;
  }

  /**
   * Makes a <em>fixed arity</em> method handle which is otherwise
   * equivalent to the current method handle.
   * <p>
   * If the current method handle is not of
   * {@linkplain #asVarargsCollector variable arity},
   * the current method handle is returned.
   * This is true even if the current method handle
   * could not be a valid input to {@code asVarargsCollector}.
   * <p>
   * Otherwise, the resulting fixed-arity method handle has the same
   * type and behavior of the current method handle,
   * except that {@link #isVarargsCollector isVarargsCollector}
   * will be false.
   * The fixed-arity method handle may (or may not) be the
   * a previous argument to {@code asVarargsCollector}.
   * <p>
   * Here is an example, of a list-making variable arity method handle:
   * <blockquote><pre>{@code
   * MethodHandle asListVar = publicLookup()
   * .findStatic(Arrays.class, "asList", methodType(List.class, Object[].class))
   * .asVarargsCollector(Object[].class);
   * MethodHandle asListFix = asListVar.asFixedArity();
   * assertEquals("[1]", asListVar.invoke(1).toString());
   * Exception caught = null;
   * try { asListFix.invoke((Object)1); }
   * catch (Exception ex) { caught = ex; }
   * assert(caught instanceof ClassCastException);
   * assertEquals("[two, too]", asListVar.invoke("two", "too").toString());
   * try { asListFix.invoke("two", "too"); }
   * catch (Exception ex) { caught = ex; }
   * assert(caught instanceof WrongMethodTypeException);
   * Object[] argv = { "three", "thee", "tee" };
   * assertEquals("[three, thee, tee]", asListVar.invoke(argv).toString());
   * assertEquals("[three, thee, tee]", asListFix.invoke(argv).toString());
   * assertEquals(1, ((List) asListVar.invoke((Object)argv)).size());
   * assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString());
   * }</pre></blockquote>
   *
   * @return a new method handle which accepts only a fixed number of arguments
   * @see #asVarargsCollector
   * @see #isVarargsCollector
   */
  public MethodHandle asFixedArity() {
    assert (!isVarargsCollector());
    return this;
  }

  /**
   * Binds a value {@code x} to the first argument of a method handle, without invoking it.
   * The new method handle adapts, as its <i>target</i>,
   * the current method handle by binding it to the given argument.
   * The type of the bound handle will be
   * the same as the type of the target, except that a single leading
   * reference parameter will be omitted.
   * <p>
   * When called, the bound handle inserts the given value {@code x}
   * as a new leading argument to the target.  The other arguments are
   * also passed unchanged.
   * What the target eventually returns is returned unchanged by the bound handle.
   * <p>
   * The reference {@code x} must be convertible to the first parameter
   * type of the target.
   * <p>
   * (<em>Note:</em>  Because method handles are immutable, the target method handle
   * retains its original type and behavior.)
   *
   * @param x the value to bind to the first argument of the target
   * @return a new method handle which prepends the given value to the incoming argument list,
   * before calling the original method handle
   * @throws IllegalArgumentException if the target does not have a leading parameter type that is a
   * reference type
   * @throws ClassCastException if {@code x} cannot be converted to the leading parameter type of
   * the target
   * @see MethodHandles#insertArguments
   */
  public MethodHandle bindTo(Object x) {
    x = type.leadingReferenceParameter().cast(x);  // throw CCE if needed
    return bindArgumentL(0, x);
  }

  /**
   * Returns a string representation of the method handle,
   * starting with the string {@code "MethodHandle"} and
   * ending with the string representation of the method handle's type.
   * In other words, this method returns a string equal to the value of:
   * <blockquote><pre>{@code
   * "MethodHandle" + type().toString()
   * }</pre></blockquote>
   * <p>
   * (<em>Note:</em>  Future releases of this API may add further information
   * to the string representation.
   * Therefore, the present syntax should not be parsed by applications.)
   *
   * @return a string representation of the method handle
   */
  @Override
  public String toString() {
    if (DEBUG_METHOD_HANDLE_NAMES) {
      return "MethodHandle" + debugString();
    }
    return standardString();
  }

  String standardString() {
    return "MethodHandle" + type;
  }

  /**
   * Return a string with a several lines describing the method handle structure.
   * This string would be suitable for display in an IDE debugger.
   */
  String debugString() {
    return type + " : " + internalForm() + internalProperties();
  }

  //// Implementation methods.
  //// Sub-classes can override these default implementations.
  //// All these methods assume arguments are already validated.

  // Other transforms to do:  convert, explicitCast, permute, drop, filter, fold, GWT, catch

  BoundMethodHandle bindArgumentL(int pos, Object value) {
    return rebind().bindArgumentL(pos, value);
  }

  /*non-public*/
  MethodHandle setVarargs(MemberName member) throws IllegalAccessException {
    if (!member.isVarargs()) {
      return this;
    }
    Class<?> arrayType = type().lastParameterType();
    if (arrayType.isArray()) {
      return MethodHandleImpl.makeVarargsCollector(this, arrayType);
    }
    throw member.makeAccessException("cannot make variable arity", null);
  }

  /*non-public*/
  MethodHandle viewAsType(MethodType newType, boolean strict) {
    // No actual conversions, just a new view of the same method.
    // Note that this operation must not produce a DirectMethodHandle,
    // because retyped DMHs, like any transformed MHs,
    // cannot be cracked into MethodHandleInfo.
    assert viewAsTypeChecks(newType, strict);
    BoundMethodHandle mh = rebind();
    assert (!((MethodHandle) mh instanceof DirectMethodHandle));
    return mh.copyWith(newType, mh.form);
  }

  /*non-public*/
  boolean viewAsTypeChecks(MethodType newType, boolean strict) {
    if (strict) {
      assert (type().isViewableAs(newType, true))
          : Arrays.asList(this, newType);
    } else {
      assert (type().basicType().isViewableAs(newType.basicType(), true))
          : Arrays.asList(this, newType);
    }
    return true;
  }

  // Decoding

  /*non-public*/
  LambdaForm internalForm() {
    return form;
  }

  /*non-public*/
  MemberName internalMemberName() {
    return null;  // DMH returns DMH.member
  }

  /*non-public*/
  Class<?> internalCallerClass() {
    return null;  // caller-bound MH for @CallerSensitive method returns caller
  }

  /*non-public*/
  MethodHandleImpl.Intrinsic intrinsicName() {
    // no special intrinsic meaning to most MHs
    return MethodHandleImpl.Intrinsic.NONE;
  }

  /*non-public*/
  MethodHandle withInternalMemberName(MemberName member, boolean isInvokeSpecial) {
    if (member != null) {
      return MethodHandleImpl.makeWrappedMember(this, member, isInvokeSpecial);
    } else if (internalMemberName() == null) {
      // The required internaMemberName is null, and this MH (like most) doesn't have one.
      return this;
    } else {
      // The following case is rare. Mask the internalMemberName by wrapping the MH in a BMH.
      MethodHandle result = rebind();
      assert (result.internalMemberName() == null);
      return result;
    }
  }

  /*non-public*/
  boolean isInvokeSpecial() {
    return false;  // DMH.Special returns true
  }

  /*non-public*/
  Object internalValues() {
    return null;
  }

  /*non-public*/
  Object internalProperties() {
    // Override to something to follow this.form, like "\n& FOO=bar"
    return "";
  }

  //// Method handle implementation methods.
  //// Sub-classes can override these default implementations.
  //// All these methods assume arguments are already validated.

  /*non-public*/
  abstract MethodHandle copyWith(MethodType mt, LambdaForm lf);

  /**
   * Require this method handle to be a BMH, or else replace it with a "wrapper" BMH.
   * Many transforms are implemented only for BMHs.
   *
   * @return a behaviorally equivalent BMH
   */
  abstract BoundMethodHandle rebind();

  /**
   * Replace the old lambda form of this method handle with a new one.
   * The new one must be functionally equivalent to the old one.
   * Threads may continue running the old form indefinitely,
   * but it is likely that the new one will be preferred for new executions.
   * Use with discretion.
   */
    /*non-public*/
  void updateForm(LambdaForm newForm) {
    assert (newForm.customized == null || newForm.customized == this);
    if (form == newForm) {
      return;
    }
    newForm.prepare();  // as in MethodHandle.<init>
    UNSAFE.putObject(this, FORM_OFFSET, newForm);
    UNSAFE.fullFence();
  }

  /**
   * Craft a LambdaForm customized for this particular MethodHandle
   */
    /*non-public*/
  void customize() {
    if (form.customized == null) {
      LambdaForm newForm = form.customize(this);
      updateForm(newForm);
    } else {
      assert (form.customized == this);
    }
  }

  private static final long FORM_OFFSET;

  static {
    try {
      FORM_OFFSET = UNSAFE.objectFieldOffset(MethodHandle.class.getDeclaredField("form"));
    } catch (ReflectiveOperationException ex) {
      throw newInternalError(ex);
    }
  }
}
